Process for carburizing and hardening



Patented July 2, 195

UNITED STATES PATENT OFFICE PROCESS FOR. cannumzmc AND HARDENING No Drawing.

Application July 12. 1930, Serial No.4 67,613

11 Claims. (01.148-15) e invention relates to the carburization and hardening of iron, mild steel and other iron a1- oys..

It is known that articles of iron, mild steel 5 and similar alloys can be cemented or carburized by treating them in liquid melts containing alkali metal cyanide by the introduction of carbonand nitrogen. The fused baths which are utilized for this purpose generally contain considerl0 able quantities of. alkali metal cyanides, such as, for instance, 4075% sodium cyanide. By the application of such melts hardening depths of about 1 mm. have been obtained. This hardening depth of the treated articles, however, can be increased only very slightly beyond this degree even by immersions of very long duration.

We havefound that we can obtain a consid- .erably better effect by using a melt which consists either entirely or to a great extent of barium chloride and adding to the same the cyanide necessary forefiecting the cementation in the form of an alkali metal cyanide.

In carrying out our invention we have foun that the. addition to the barium chloride melt of a few percent of alkali metal cyanide is suflicient to produce an excellent cementation effect. The quantity of sodium cyanide to be added may be for instance 1-3%, but it may vary and be slightly higher orlower. Even on adding only 1% and less of sodium cyanide cementations have been obtained which are of a very satisfactory depth.

. gTlie barium chloride melt containing cyanide according to our invention has a very long life and will give excellent results unto the last, which is the more surprising as by the use of, for instance, calcium chloride instead of barium chloride a rapid and far-reaching decomposition of the cyanide takes place with a simultaneous generation of carbon and a sludge or scum formed of lime.

For carrying out our process we have found that on the whole temperatures of from 850 to about 950 centigrade are the most suitable. The temperature most suitable for each case depends 'upon the properties of the materials to be carburlzed or cemented, upon the kind and depth of the hardening desired and so on. and can easily be ascertained by preliminaryexperiment. It is advantageous to add to the barium chloride melt containing cyanide some other, salts in order to reduce. the fusion point of the mixture, since it is desirable to make the melt as thin as possible,

considerably favoured, so that either the durathus avoiding unnecessary losses. These losses remit/chiefly from the 'salt adhering to the articles casehardened in the bath and removed with them. The salts decreasing the fusion point of the barium chloride are for instance sodium chloride, potassium chloride or the like, or mixtures of the same.

We have found it advantageous to keep the air away from the surface of the contents of the crucible or vessel containing the bath. This can be done, for instance, by covering the crucible or vessel afterthe articles to be casehardened have been immersed in the salt melt with a wellfitting lid or for instance by covering up the surface of the salt melt with a layer of graphite, wood charcoal or other substances which prevent the oxidation of the crucible contents; both measures may also be applied together.

Example 1 Iron bolts with a carbon content of 0.1% are immersed in a fused bath kept at 920 0., this 2 bath being made up by mixing parts of barium chloride; 25 parts sodium chloride and 25 parts potassium chloride and adding 3 parts sodium cyanide. After an immersion of 2 hours the visible depth of -cementation was 1 mnn, after 4 hours 1.3-1.4 mm. The transition of the cemented or 'casehardened layer into the uncemented one, when carrying out the casehardening according to the process of our invention, is a very gradual one. At .the same time this cemented layer after the article has been quenched shows glass hardness toa considerable depth. In consequence several tenths of a millimetre may be ground off and the surface of the article remaining is still as hard as glass. If-

the melt becomes less eflicacious in the course of time, the eflicacy may be restored by'adding some more alkali metal cyanide. The fresh alkali metal cyanide may be added either in very small quantities at a time, corresponding to the gradual decrease of the eficiency of the bath, or greater quantities may be added after long periods of carburization. Moreover, we have found that the cementation according to our invention may be favourably influenced and considerably increased by providing for the presence in the melt of small quantities of finely divided carbon, such as wood charcoal, activated charcoal or the like. The permeation of the carbon is thereby tion of the immersion may be shortened if a case of a specified depth is desired, or in acertain unit of time greater depthsmf peneia'ation are obtained than when :working without theaddition of carbon. The carburized layer is eutectic even in the presence of finely distributed coal. Secondly considerable parts of it show glass hardness after quenching. Here also the transition of the carburized layer to the uncarburized one is gradual, therefore a cracking off of the carburized layer need not be feared.

The quantity of the finely distributed coal is advantageously kept within such limits that the bath remains as thinly fluid as possible and its fluidity is not interfered with by the presence of the carbon, in order that the fused salt runs off the carburized article after its withdrawal from the immersion bath as thoroughly as possible. In most cases fractions of 1% of carbon such as 0.1-0.9% are suflicient to produce a considerable acceleration of the immigration of the carbon. The coal is added by introducing it in a finely divided, for instance, finely ground form into the melt. It may, for example, be added in conjunction with the cyanide with which it may be fused together beforehand. The finely distributed coal may also be generated'in the melt itself by adding carbonaceous substances such as sugar, sawdust or the like, which in consequence of the heat and the action of the salts are carbonized.

y 4 Example 2 The melt consists of 60 parts barium chloride, 20 parts sodium chloride and 20 parts of potassium chloride, to which are added 3 parts potassium cyanide and 0.2 parts of finely ground wood charcoal. Into this bath iron bolts of a carbon content of 1% are introduced and kept for 4 hours at 930 C. After this time the, visible depth of case is 1.4-1.6 mm., about half of which shows glass hardness after quenching.

The fusion melt, according to our invention, has the special advantage that no sludge or sediment is formed during its use, which might prevent a uniform cementation of the articles immersed or the uniform heating of the'crucible contents. These sediments require special precautions in order to keep up an even efilciency of the bath, such precautions being, for instance, the removal from time'to time of the sediment formed.

Carburization baths accordingto our inventionhave furnished results which could hitherto not be obtained by any other immersion method but only by casehardening in powdered material. This casehardening in powder has the wellknown drawbacks that it is scarcely possible to obtain uniform temperatures in the boxes used for the casehardening, moreover it takes a very long time to heat these boxes to the desired temperatures, and furthermore, the duration of time required for obtaining a certain specified depth of carburization is three or four times longer than when using the process of our invention.

Further investigations have shown that the process according to our invention may be car-' ried out with special advantage by using in addition to those inert salts which reduce the viscosity of the melt strontium salts, such as for instance strontium chloride. The fusion point of the melt is reduced on the one hand, so that the melt becomes more liquid at the temperature of application. On the other hand, at the temperatures utilized for the carburization the speed of decomposition of the cyanide is not inconsiderably enhanced when strontium salts are present compared with a bath in which strontium salts are absent and barium chloride alone is used. In consequence, by varying the quantities of strontium salts added to the barium chloride melt, we are able to regulate at will the decomposition in the bath and thereby also the speed and the intensity of the carburization.

Example 3 A melt which was found to be of an excellent efficiency consisted of 50 parts barium chloride, 20 parts strontium chloride, 10 parts sodium chloride, 10 parts potassium chloride and 10 parts sodium cyanide.

In a bath of this description it is advantageous, since the cyanide decomposes very rapidly, to increase the quantity of the alkali metal cyanide added, as we have done in the above example.

A fusion bath of this description was still very eflicient after it had been in use for several days. When the speed of carburization slackened, we were able to restore its former efficiency by adding small quantities of alkali metal cyanide either in an undiluted form or in mixture with other salts.

The articles carburized and cemented according to our invention are quenched in the usual way.

What we claim is:

1. Process of carburizing articles made of iron or its alloys which consists in immersing said articles in a molten bath consisting essentially in addition to barium chloride of substances reducing the fusion point of the melt, selected from the group comprising sodium chloride, potassium chloride, and strontium chloride, to which from 1 to 10 percent alkali metal cyanide have been added, at temperatures between 850 to 950 centigrade, and quenching same.

2. Process of carburizing articles made of iron or its alloys which consists in immersing said artides in a fused bath consisting essentially of barium chloride, strontium chloride, and substances selected from the group comprising sodium chloride and potassium chloride reducing the melting point of the mixture to which from 1 to 10 percent alkali metal cyanide have been added, at temperatures between 850 to 950 centigrade and quenching the same.

3. Proces of carburizing articles made of iron or its alloys which consists in immersing said articles in a fused bath consisting essentially of 40 to 100 parts of barium chloride, 1 to 60 parts of strontium chloride and substances selected from the group comprising sodium chloride and potassium chloride reducing the melting point of the mixture, to which 1 to 10 parts of alkali metal cyanides have been added and in which a small quantity of finely divided carbon is distributed, at temperatures between 850 to 950 centigrade,and quenching the same.

4. Carburizing bath for articles made of iron and iron alloys consisting essentially of 40 to 100 parts of barium chloride, 1 to 60 parts of substances selected from the group comprising sodium chloride, potassium chloride, and strontium chloride reducing the melting point, 1 to 10 parts of alkali metal cyanides.

5. Carburizing bath for articles made of iron and iron alloys consisting essentially of 40 to 100 partsbarium chloride, 1 to 60 parts alkali metal chlorides, 1 to 30 parts strontium chloride, 1 to 10 parts of alkali metal cyanides.

6. Carburizing bath for articles made of iron and iron alloys consisting essentially of 50 parts of barium chlorid, 50 parts of sodium and potassium chloride, 1 to 3 parts of sodium cyanide.

7. Carburizing bath for articles made of iron and iron alloys consisting essentially of 50 parts potassium chloride and 1 to 3 parts of sodiumcyanide.

8. Carburizing bath for articles made of ironand iron alloys consisting essentially of 40 to 100 parts of barium chloride, 1 to 60 parts of sub-- stances, selected from the group comprising sodium chloride, potassium chloride, and strontium chloride reducing the fustion point of the melt,

1 to 10 parts of alkali metal cyanide and up to one part of finely divided carbon. 7

9. Carburizing bath for articles made of iron and iron alloys consisting essentially of 40 to100 parts of barium chloride, 1 to parts of strontium chloride, 1 to 10 parts of alkali metal cya-' nide and up to one part of finely divided carbon. 10. Process of carburizing ferrous articles which consists in immersing said articles in a fused bath consisting essentially of 40 to-lOO parts of barium chloride, 1 to parts of substances selected from the group comprising sodium chloride, potassium chloride and strontium chloride reducing the fusion point of the bath, and 1 to 10 parts of alkali metal cyanide, removing and quenching the same.

11. Process of carburizing ferrous articles which consists in immersing said articles in a fused bath consisting essentially of 40 to parts of barium chloride, '1 to 60 parts alkali metal chlorides, l to 30 parts strontium chloride and 1 to 10 parts of alkali metal cyanides, removing and quenching the same.

CARL ALBRECHT. WALTER BECK. 

